It's already hard enough for players buried in the depth chart at higher level schools to find playing time at lower level schools. When it comes to the transfer portal, there are plenty of players who were highly recruited by the higher level programs, which makes choosing a lesser school not a sure thing for PT among the higher profile players.
Furthermore, coaches tend not to be patient, that they want to see results on the field, right away. They don't see transfer portal players as long term projects, and they want their hired guns to hit the ground running.
Players buried in the depth chart at lower level FBS schools aren't going to even be a blip on the radar, unless they're transferring to some Division II, Division III, or junior college, or if they happen to know the coach of some other lesser FBS school very well (and gets time as a personal favor).
Lesser FBS teams are looking to upgrade their roster, meaning that they're probably going to be looking at the TP players who came from the bigger programs. They're not going to shell out money for another lesser FBS school's player who hasn't shown anything, especially since they don't have nearly the same levels of NIL money that the bigger programs do.
At best? I'm guessing that the players buried in the depth chart at those lower level FBS schools would only benefit if the new school were better academically and / or provided more resources for their alumni when it comes to finding a good job.